Reduction of the size of particulate solids is an important step in many processes. For example mineral ores are frequently subjected to particle size reduction prior to further processing steps such as froth flotation, mechanical separation, and pelletization. Grinding operations are usually carried out in mills such as ball, bead, rod, stirred, attrition jet, autogenous or pebble mills depending on the degree of comminution desired. The grinding is usually accomplished in the presence of a liquid medium, which in most instances is water.
In the processing of ores, an essential step is the size reduction or comminution of the ore to the size at which valuable metal grains are released from the gangue matrix. As the quality of ore available decreases, the degree of comminution necessary to release the metal-containing grains increases. This in turn increases the grinding cost to process the ore. Since the grinding process is very energy intensive, the increases in energy costs coupled with the need for additional grinding has resulted in grinding costs becoming a significant portion of the cost of processing minerals and other materials.
The amount of breakage per unit time (breakage kinetics) and mass transfer of grinding is normally controlled by the amount of water present or the concentration of the material in the mill. Water is an excellent medium for grinding because of its high polarity. When the mass transport of aqueous slurries through the mill decreases, corrective action is taken by either increasing the amount of water or decreasing the amount of solids entering the mill. These actions avoid overloading the mill, but decrease efficiency since fewer solids are ground per unit of time.
Various chemical agents that act as grinding aids have been employed in efforts to increase grinding efficiencies and economics. One way in which grinding efficiencies may be improved is by lowering the viscosity of a slurry of a given weight percent solid, especially if the weight percent solid loading is on the high side. Reducing the viscosity allows an increase in the concentration of solids that can be ground within a given unit of time. A suitable grinding aid must meet additional requirements, since grinding is a preliminary step in processing, which include the impact of the grinding aid on subsequent operations. Various dispersants and surfactants such as anionic polyelectrolytes, polysiloxane, organosilicones, glycols, amines, graphite and non-polar liquids have all been used with varying degrees of success. Although some of these grinding aids do in fact lower viscosities, the necessary concentration at which such lowering is accomplished makes their use cost-prohibitive and/or also creates an impact on further processing of the comminuted solids.
Chemical agents to effectively act as grinding aids (1) must absorb on enough of the solid surfaces available to affect the slurry viscosity; (2) must be able to affect the viscosity at low grinding aid concentrations or high solids to grinding aid ratios; (3) must not adversely affect down-stream operations; (4) must be non-toxic and degradable; (5) must not increase and preferably decrease steel media wear resulting from corrosion or abrasion; and (6) must be able to function in hard water media commonly used in grinding operations.
Because of the large scale on which commercial grinding operations are carried out even an efficiency improvement of a few percentage points is of major economic significance.
It is an object of the present invention to provide grinding aids having the foregoing properties to an extent greater than grinding aids heretofore developed.
It is another object of this invention to provide grinding aids, which more efficiently grind minerals to a smaller size or at an increased throughput.